1. Field of the Invention
The present invention relates to switching laser comprising, in a cavity, a cell of an amplifying gas and a cell of an absorption gas, means for exciting the amplification gas and means for exiting the absorption gas.
2. Description of the Prior Art
When a cell containing an absorbent gas is placed in the cavity of a laser, the normally continuous laser emission may be converted into a pulsed emission, providing that the absorber is a saturable absorber whose saturation parameter is comparable to the intensity flowing in the laser.
This conversion has been designated as "passive Q-switching" where Q is the quality coefficient.
For pulsing for example a CO.sub.2 laser, as early as 1967 the use of sulphur hexafluoride, propylene, formic acid, and even carbonic gas, or a mixture containing this gas, have been proposed depending on the wave length of the laser emission.
When carbon gas is used as pulsing agent for a CO.sub.2 laser, the laser absorption transition and emission transition bring into play the same vibratory levels. Furthermore, the pulsing carbonic gas must be heated for populating the absorption level, this gas not being absorbent at its fundamental level.
It should be mentioned here that the passive switching of a CO.sub.2 laser by the same CO.sub.2 does not require a network in the cavity since, not only for amplification but also for absorption, the transition takes place between the same two levels, and the problem of coincidence of the emission and absorption spectral lines does not then arise.
Generally, the rate of the pulses of passive switching laser depends on the nature of the switching, or pulsing, gas, on its absorption coefficient, on its temperature, on its pressure and on the light intensity of the laser.
The passive switching of a laser is explained in the following way:
A spontaneous and stimulated emission arrives on the absorbent gas. This latter first of all resists the bleaching of this emission, particularly by transfer of collision energy at the absorption level. The laser emission is thus prevented. When the spontaneous and stimulated emission becomes sufficiently intense and finally succeeds in passing through the absorber, this emission introduces a transparency state, the population at the two levels of the transition of the absorbent being the same, and the emission becomes a laser emission. The laser then releases all its energy; during the pulsing, and after relaxation of the molecules, the lower level of the transition of the absorber is repopulated until it becomes opaque again and this phenomen continues again and so on.
The construction of passive switching lasers has been known for a long time, which comprise, in a cavity, an amplification cell and an absorption cell, containing for example CO.sub.2 or a mixture of CO.sub.2 and another gas, a DC power source for amplification and a heating source for the absorption.
However, the rate of the pulses emitted by passive switching lasers constructed at the present is not strictly constant, a random shift or jitter between the successive pulses occurring from one period to another because of the random start up of the laser emission.
Although it has already been proposed to regulate the average rate of the pulses of a passive switching laser by causing the light intensity of a laser for example, to vary, this regulation by varying the intensity has never overcome this jitter question.
It is this question which the present invention proposes dealing with.
The document Applied Physics B. Photophysics and Laser Chemistry, vol. 830, no. 2, February 1983, Heidelberg (DE), E. Arimondo et al: "Repetitive passive Q-switching and bistability in lasers with saturable absorbers", pages 57-77, it is true already teaches the exciting of the gas of an absorption cell by using a radio frequency DC voltage source. However, the authors of the article whose title is moreover significant, have never sought to resolve this question of jitter. These authors were in fact only concerned with the transitory phenomena due to the sudden variations of the populations of a molecule taking part in an absorbent transition for the laser radiation, inducing power modulation of the laser which is damped. In other words, they analyzed how the switching mode was reached, where the damping times become infinite. But the work reported in this document remained limited to passive switching lasers.
The problem of controlling the switching of lasers remains then unsolved and it is this problem which the applicant has sought to solve.